The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Global climate change (i.e., global warming) is believed to be caused by anthropogenic emissions of greenhouse gases. Modeling of global warming effects predicts global increases in temperature and sea levels, shifts in weather patterns, and more extreme weather events, including flooding and droughts. Greenhouse gases include carbon dioxide, methane, nitrous oxide, water vapor, ozone, and perfluorocarbons/chlorofluorocarbons. It has been estimated that carbon dioxide accounted for about 84% of greenhouse gas emissions in the United States in 2000. The rate of emissions of carbon dioxide (CO2) and other hazardous air pollutants is highly correlated to both economic and industrial growth and has increased significantly since the mid-1800s. CO2 is typically generated by combustion of hydrocarbons, fossil fuels and/or by various industrial processes that generate carbon dioxide byproduct, including in cement, lime, iron, and steel manufacturing. The United States Environmental Protection Agency (EPA) and the United Nations Intergovernmental Panel on Climate Change (IPCC) classify emissions based on fuel combustion (which predominantly includes motor vehicle and power plants) and other industrial sources. 97% of anthropogenic CO2 emissions in the United States are attributed to fossil-fuel combustion sources, such as power plants, incinerators, and motor vehicles. Other significant point sources of carbon-dioxide include cement, lime, and iron/steel manufacturers, all of which generate copious CO2 during processing, both as a reaction byproduct and through burning of hydrocarbon fuels.
In addition to being an undesirable greenhouse gas, CO2 has the potential to create operational and economic issues, as it is a diluent without any fuel value. It is an acid gas and can cause corrosion problems in the presence of water, creating carbonic acid that can be quite corrosive to some alloys.
Through international treaties, such as the Kyoto Protocol, numerous nations have committed to reducing emissions of various greenhouse gases, including CO2. In the United States, there has traditionally been a great focus on developing equipment to effectively reduce emissions of regulated air pollutants, such as particulate matter, sulfur oxides, and nitrogen oxides. However, development of abatement technology for unregulated CO2 emissions has lagged behind other control technology. However, as various nations implement regulations and trading programs that restrict the generation of various greenhouse gases, in particular CO2, there is an emerging need for more effective and inexpensive CO2 abatement technologies.
Existing methods for the removal of CO2 from gas streams include chemical absorption/adsorption with particular solvent systems (amine scrubbing), membrane separation, cryogenic fractionation, and/or adsorption using molecular sieves. In disposable systems, the active material(s) will make a single pass through the reactor/scrubber and is then discarded. One-time use systems are less desirable due to the added expense and maintenance associated with the disposal of larger amounts of spent active material. Regenerative systems are designed to regenerate the active material, making it suitable for subsequent productive passes through the reactor. Molecular sieves, such as zeolites and activated carbon, are used in regenerative pressure swing adsorption (PSA) or temperature swing adsorption systems which separate gas mixtures by selective adsorption of one or more of the gases at high pressure and/or low temperature, to remove the undesirable components from a gas stream. The captured impurities are then desorbed by lowering the pressure, or increasing the temperature, of the adsorbent system (thus the system “swings” from a high to low pressure or a low to high temperature). The desorption step regenerates the adsorbent material for reuse during the subsequent adsorption step. PSA systems typically comprise several adsorption beds, through which the gas stream is passed, allowing for separation of the selected gas species. Each of the above processes has drawbacks, including high capital investment and operating costs, as well as relatively small throughput capacity and low removal efficiency in some cases. Such systems are potentially cost-prohibitive for various applications, in particular for high throughput manufacturing facilities that generate high quantities of carbon dioxide and other emissions.
Thus, there is a need for processes that reduce CO2 emissions from exhaust gases of stationary sources in an efficient manner and further, are cost-effective. Additionally, CO2 emission abatement equipment can preferably handle high flow rates associated with industrial applications while achieving desirable removal efficiencies. Preferably, such abatement processes are regenerative and employ recycling to embody sustainable development initiatives.